Remediation: Techniques and Cost - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 388

Treatment
plant

Clean
water

Air pollution
control
equipment

Wash
water

Gases

Separator/
washer

Dewatering
system

Reactor

Polluted
soil (sifted)

APEG

Clean soil
(returned to site)

FIGURE 11.12
Example of the glycolate dehalogenation process. (From United States Environmental Protection Agency
(USEPA), A Citizen's Guide to Chemical Dehalogenation , EPA 542-F-01-0010, Office of Solid Waste and Emergency
Response, Washington, DC, 2001g.)

This sifted soil is then mixed with chemical agents and heated in a reactor. During this
process, a chemical reaction occurs that removes the halogen atom from the molecular
structure of the contaminant, and depending on the completeness of the reaction, destroys
or renders the contaminant less harmful.
There are two common types of chemical dehalogenation: glycolate dehalogenation
and base-catalyzed dehalogenation (USEPA 2001g). Figure 11.12 is an example of glycolate
dehalogenation.

11.4 Summary of Soil Remediation Technologies
Each soil remediation technology described in Section 11.3 has advantages and disadvan-
tages depending upon the different conditions existing at any given site. Frequently, a
combination of technologies is used to achieve the objective. At sites where there are wide-
spread impacts with different types of contaminants, rarely if ever, is one technology used
(USEPA 2007).
Figure 11.13 summarizes the different soil remediation technologies applied at 977
CERCLA Superfund sites in the United States (USEPA 2007). A total of 53% of the tech-
nologies used ex situ technologies and 47% used in situ technologies. Soil excavation
was excluded because it was the technique most commonly employed, and it remains
the most common technology used for remediation of soil. This prevalence exists in part
because soil excavation is the least costly method when relatively small volumes (less than
5000 ton) of contaminant are present and accessible. However, in a recent study of metal-
contaminated soil in southeast Michigan, Murray et al. (2004) discovered that the most
extensive contamination is restricted to the upper 0.5 m of soil, thus making soil excava-
tion not only cost effective, but quite effective at quickly and efficiently removing the soil
posing the greatest risk to human health. In addition, soil excavation may be the only eco-
nomical technology available, especially when different types of contaminants are present
(USEPA 2007; Rogers et al. 2009).